In compression molding, a lump or slab of preheated thermoplastic material may be pressed between two matched mold halves that compress the thermoplastic material into a desired part or shape. The mold is closed and pressure is applied to force the thermoplastic material into contact throughout the mold, and heat and pressure are maintained until the thermoplastic material has cured. The process employs thermosetting resins in a partially cured stage, either in the form of granules, putty-like masses, or preforms.
Compression molding is a high-volume, high-pressure method suitable for molding complex, high-strength fiberglass reinforcements. Advanced composite thermoplastics can also be compression molded with unidirectional tapes, woven fabrics, randomly orientated fiber mat or chopped strands. The advantage of compression molding is its ability to mold large, fairly intricate parts. Compression molding produces fewer knit lines and less fiber-length degradation than injection molding.
However, a disadvantage of compression molding is that a high capital investment is required to purchase high capacity presses capable of providing 2000-3000 tons of pressure, and high-pressure molds. This is really only efficient for large production volumes. Lower volumes of smaller parts can be manufactured using aluminum molds on existing presses to save some cost. Other disadvantages of the process are low fiber fractions due to viscosity problems.
An alternative to compression molding is disclosed in U.S. Pat. No. 6,900,547 in which gravity injection of thermoplastic material is used in the compression molding process. In particular, the thermoplastic material is gravitated onto a bottom half of a mold as it is moving toward a stationary top half of the mold. The '547 patent is hereby incorporated herein by reference in its entirety.
In U.S. Pat. No. 6,126,433 a molding material is plasticized in an extruder unit and is introduced into a compression mold unit by an injection unit. The extruder and one or more injection units are independent units arranged on a coordinate table. The extruder and injection units can be separated from and joined to one another such that one or more injection units can be releasably coupled to an extruder unit, then uncoupled from the extruder unit and moved toward one or more compression molding units. Because the injection unit can be uncoupled from the extruder, only the significantly lower mass of the injection unit has to be moved in order to deposit the thermoplastic material inside the mold.
Even in view of the various alternatives to compression molding, there is still a need to improve this process.